1. Field of the Invention
This Invention relates to structures and methods necessary to evaluate the effects of variations in process parameters during the fabrication of integrated circuits upon a semiconductor substrate and more particularly to structure and method to determine the density of charge induced by radiation during such processes as ion implantation and plasma etching, to evaluate subsequent damage to the semiconductor substrate from the radiation induced charge.
2. Description of Related Art
During the fabrication of integrated circuits the surface of semiconductor substrates are exposed to sources of high energy radiation. An example of such a source is ion implantation, where high concentrations of elemental materials such as boron are forced to diffuse into the surface of the semiconductor substrate. Another example is plasma etching, where material is removed from the surface of the semiconductor substrate by the action of a high energy plasma eroding the surface of the semiconductor substrate.
In each of these examples, electrical charges can accumulate at the interface between a conductive material such as a metal and an insulator such as silicon dioxide. If the magnitude of the charge is sufficiently large the insulator may be damaged.
Referring to FIG. 1a, a semiconductor substrate 10 has a well 12 of a first conductivity type such a p-type material diffused into the surface of the semiconductor substrate 10. A drain region 14 and a source region 16 is diffused into the well 12 in the surface of the semiconductor substrate 10. Metal is deposited upon the surface of the semiconductor substrate 10 to form a drain contact 18 and source contact 20. A thin gate oxide 22 is grown between the drain region 14 and the source region 16. A field oxide 24 is grown on the surface of the semiconductor substrate 10 in the area outside the drain region 14 the source region 16, and the thin gate oxide 22. A polysilicon gate contact 28 is opened in the field oxide 24.
This structure is a thin oxide field effect transistor (FET) and is constructed by processing that is well known and practiced in the art.
To evaluate the charge effects of radiation during the semiconductor processing, a relatively large area of metal 26 is formed on the semiconductor substrate and connected to the gate 28 of the FET. The meal area 26 acts as the top plate of a charge collection capacitor with the field oxide 24 being the insulator and the semiconductor substrate 10 being the bottom plate. The capacitance of this structure is directly proportion to the area of the top plate metal 26.
FIG. 1b is a view of the cross section of FIG. 1a at line 1b-1b'. If this structure is exposed to a radiation source 30, electrical charges 32 will collect at the surface interface between the metal layer 26 and 28 and the field oxide 24 and the thin gate oxide 22. As is shown in FIG. 1c, which is a cross section of FIG. 1a along the line 1c-1c', the induced charges 32 will create an electrical field 34 within the thin gate oxide 34 sufficiently large as to damage the thin gate oxide 34.
The magnitude of this voltage V.sub.b can be calculated as: ##EQU1## where V.sub.b is the magnitude of the voltage at the gate 34;
Q.sub.collect is the magnitude of the charge 32; PA1 C.sub.1 is the magnitude of the thin oxide capacitance between the gate metal 28 and the well 12 within the substrate 10; and PA1 C.sub.2 is the magnitude of the charge collection capacitor 26.
FIG. 2 shows a schematic of the equivalent circuit for the device of FIG. 1a. While an estimate of the charge Q.sub.c maybe made, there is no retention of the charge Q.sub.c once the radiation producing processing step is completed. The charge Q.sub.c will be dissipated during the handling and further processing of the semiconductor substrate.
Traditional methods for evaluation of radiation induced charge damage to a semiconductor substrate have required the cross-sectioning of the semiconductor substrate and examining the cross-sectioned surface in the area of the thin gate oxide 22 of FIG. 1a and the field oxide 24 of FIG. 1a. The level of damage is an indicator of the level of the charge induced during the processing step, but can not provide any direct measurement of the charge density and consequent field strength
U.S. Pat. No. 5,396,169 (Buehler, et al.) describes a method for predicting the single event upset susceptibility of a static random access memory to alpha particle radiation.
U.S. Pat. No. 4,196,389 (Kelly, et al.) discloses a test site structure to evaluate the minimum distances between adjacent channels of charge coupled device. The test site will allow measurement of charge transfer efficiency and charge leakage between channels.